The mammalian spermatozoa is a unique cell type in which the DNA is arranged into a compact and genetically inert state. Recently, the existence of a well-defined spatial organization of genome - genome architecture (GA) have been shown in spermatozoa of humans: each chromosome occupies distinct territory, all centromeres are clustered into a compact chromocenter buried within the nucleus, while all telomeres are exposed at the periphery and interact to form dimers. The general hypothesis is that non-random sperm GA determines and promotes an ordered chromosome withdrawal and movements during successful fertilization. In particular, defective GA in some cases of (male infertility may have an adverse impact on early development. The long-term goal is to establish the "standard" of how chromosomes are packed and arranged in normal, fertile human sperm, and to develop in situ tests for proper selection of donor cells during assisted reproduction. The first objective is to unravel as yet unknown characteristics of GA in sperm of donors with proven fertility. In preliminary experiments a new approach to directly observe chromosome organization in sperm has been developed. In the proposed study, nuclear path and higher- order structures of individual chromosomes will be established. Next, the nature of telomere dimers will be analyzed. Preliminary data indicate that these are formed by interactions between two ends of one chromosome. If so, chromosomes in human sperm nuclei are looped. Further, nuclear positioning of individual chromosomes will be determined. Strong indications towards non-random spatial arrangement of chromosomes in sperm nuclei have been obtained in preliminary studies. The second objective is a systematic study of GA in spermatozoa of men with abnormal sperm morphology undergoing treatment with IVF/ICSI. These studies will establish how chromosomes are packed and arranged in fertile human sperm and will lead to a better understanding of the role played by the male genome spatial organization at fertilization. The information obtained should contribute to an improvement of diagnostic assays and better prediction of the outcome of assisted reproduction. Finally it will add to our basic knowledge about structural organization of chromosomes in human cells.